>But I cannot see how there can be an "uncancelable" meaning to a
>single word or a grammatical form.

Example: in English, the simple past tense has past reference. This past
reference is not cancelable, which is why you can't say this:

*tomorrow I went to the store

And if you say this:

today I went to the store

It is clear that you have already gone to the store, even though the rest
of the sentence could just as easily be wrapped around another verb form to
indicate future reference:

today I am going to the store

I don't think you can find a context that will cancel the past reference of
"went" to make it refer to present or future time. I've been wrong before!
Similarly, I believe the word "yesterday" has a non-cancellable past
reference.